Specifically, the reserve capacity is how many minutes (or hours) the battery will maintain a useful voltage (eg 10.5v for a car battery) if a constant draw of 25 amps is applied.

This is useful for the typical applications of deep cycle batteries, which frequently are used for long periods of time at a constant rate.

In the case of a 12v deep cycle battery, the reserve capacity is also conveniently how long the battery can provide a quarter of a kilowatt of AC power by way of a DC/AC inverter. Arrayed in groups of four, which also minimizes the Peukert Effect, the reserve capacity is kilowatt/minutes (or kilowatt hours, with a simple conversion.)